1. Field of the Invention
The invention relates to a method and apparatus for isolating a production zone in a subterranean well when tubing is retrieved therefrom without killing the well or encountering liquid backflow through the tubing as it is retrieved.
2. Description of the Prior Art
In order to recover oil in production zones after economical primary production operations have terminated, produced salt water from other zones or reservoirs is injected into the production zone by means of injection wells to cause migration of the remaining oil to the producing well. This water may be expected to have a high solids content and, often, because of exposure to air at the well surface, the water also will contain dissolved oxygen, the combined features of dissolved oxygen in the water and salt water being utilized as the liquid injection medium defining a highly corrosive environment for the tubing.
In order to recover the remaining oil in the production zone at an economical rate, the flow rate is increased by maintaining a high water injection pressure which also may ultimately increase the initial and normal pressure of the production zone in the vicinity of the injection well bore.
Because of the high pressure and high corrosive environment, it is desirable to run tubing within the casing, the tubing carrying a packer apparatus for sealing engagement along the I.D. of the casing within the tubing-casing annulus above the injection zone to completely isolate the annular area thereabove from the injection fluids. The packer apparatus normally is designed to selectively latch and sealingly receive the bottom exterior end of the tubing string, so that it may be removed from the packer apparatus, leaving the packer apparatus sealingly intact.
Oftentimes, because of the increased production zone pressure, the injected fluid will backflow through the upper end of the tubing at the top of the well as it is being retrieved. If precautions are not taken, the backflow will result in the need for auxiliary disposal means and procedures to avoid potential environmental damage in the vicinity of the well site. Lack of precautions also may result in loss of oil production through the production well, since the injected salt water fluid is reversed by the backflow, thus reducing the migration of the production toward the production well.
Typically, backflow problems have been avoided by seating of a wire line blanking plug into the bore of an on-off sealing connector, the plug preventing flow of fluid in either direction so that the tubing string may be pulled out of the well bore and rerun without backflow of the injected salt water. Such a procedure is not entirely satisfactory because it may be assumed that the tubing string has suffered considerable chemical deterioration resulting from oxygen corrosion and scale deposits, such that the time consuming and costly running and sealing engagement of a plug by wire line cannot be reliably accomplished. Alternatively, a weighted fluid may be injected into and circulated through the tubing-casing annulus into the well bore above the packer to balance the reservoir pressure and thus prevent a salt water backflow. This procedure is quite costly, is time consuming, and can lead to additional problems when the weighted fluid is removed from the well, i.e., problems encoutered in preventing inadvertent injection of the weighted fluid into the formation.
Similar problems and solutions as those set forth above are found in salt water disposal operations. Additionally, the need to retrieve tubing strings is encountered in artificial lift wells in which gas lift mandrels must be relocated. If the well is capable of flowing at a reduced rate without artificial lift applications, a means for isolating the flow of fluid through the tubing string must be utilized.
In all of the above circumstances, a safety device is necessary to prevent any fluid flow due to unforeseen circumstances while the tubing is being retrieved and subsequent thereto.
The present invention obviates the problem set forth above by providing a method and apparatus for selectively retrieving a tubing string, the lower opened end of which is sealingly insertable through the bore of a packer which, in turn, is sealingly engaged onto the inner wall of the casing above an injection or other zone, the tubing retrieval being accomplished with mechanical valve means being provided to isolate the well flow therethrough and prevent backflow of the injection medium.